Head mount displays (HMDs) that are mounted on the heads of users have become known in recent years. A head mount display, in principle, is designed to enlarge an image displayed on a small-sized display device with an enlarging optical system, and lead the image to an eye of an observer. That is, a head mount display is designed to optically enlarge an image displayed on a display device, and allow a user to observe the image as an enlarged virtual image.
As this type of head mount display, an optically-transmissive head mount display designed to enable an observer to observe not only the above mentioned virtual image but also an outside image is known. This optically-transmissive head mount display is designed to superimpose an image displayed on a display device onto an outside image and lead the image to an eye of an observer with an optical system.
The visibility of a virtual image reproduced by this optically-transmissive head mount display depends on the environment in which this virtual image is displayed. For example, inconsistency between the display state of this virtual image and the state of the real world hinders comfortable observation, or the visibility of the virtual image is lowered depending on the display position thereof.
Patent Document 1 discloses adjusting the depth of an entire image by taking into account to which object the observer is paying attention, for example. Patent Document 2 discloses adjusting the disparity of a virtual image in accordance with convergence of the eyes by using an eye-gaze tracking technique, for example.